1. Field of the Invention
The present invention relates to a waveform analyzer capable of separating an eight field sequence of a phase alternation by line (PAL) video signal and accumulating data of an optional field in accordance with an internal setting.
2. Description of the Related Art
Conventionally, in a waveform analyzer of this type, an input signal is converted into a digital signal by means of an A/D converter. Further, the input signal is inputted to a TV synchronizing signal separator which is constructed so that an odd-numbered field can be separated from an even-numbered field by detecting a difference among serrated pulses at a starting point of vertical synchronization of a video signal (the input signal). The TV synchronizing signal separator outputs vertical synchronizing pulses for every odd-numbered field or even-numbered field in accordance with an internal setting determined by a state setting of the analyzer by a user from a panel surface, a GP-IB interface or the like of the analyzer. Further, the TV synchronizing signal separator separates horizontal synchronizing signals of the input signal and outputs the horizontal synchronizing pulses. The vertical synchronizing pulse and the horizontal synchronizing pulse that are outputted from the TV synchronizing signal separator are inputted to a trigger gate and selector circuit, and are selected into a trigger pulse A for starting a delay counter and a trigger pulse B used as a reference for stopping to take in the waveform. The delay counter and a timing control circuit for controlling a memory are started by means of the trigger pulses A and B, thus making it possible to collect field signals in accordance with the internal setting at a desired delay time and sampling period.
However, the conventional waveform analyzer described above has such a problem that, although it is possible to decide to which field, either an odd-numbered field or an even-numbered field, the accumulated data belong for an input signal having a plurality of field sequences such as a PAL video signal, it is impossible to decide the number of the field to which the data belong.
As a method of detecting a color field of a PAL video signal, a method of detecting a color field by adding one of a plurality of phase angles, which are predetermined respectively based on the mode of combination of a result of detecting whether it is an odd-numbered field or an even-numbered field and a result of detecting the existence of a burst signal belonging to a horizontal synchronizing pulse immediately after an equivalent pulse, to a subcarrier-to-horizontal phase (SCH phase) detected separately, and depending on whether the phase value obtained by the addition belongs to a range of 270.degree. to 90.degree. or a range of 90.degree. to 270.degree. has been disclosed in JP-A-4-96491. However, this method of detection has such a problem that the color field cannot be detected when the phase value obtained by the addition reaches 90.degree. and 270.degree., i.e., there are two undetectable points.